MD5加密

新建NSString的分类，代码如下

#import "NSString+MD5.h"
#import <CommonCrypto/CommonDigest.h>

@implementation NSString (MD5)

//32位
- (NSString *)getMd5_32Bit {
    const char *cStr = [self UTF8String];
    unsigned char digest[CC_MD5_DIGEST_LENGTH];
    CC_MD5( cStr,(integer_t)self.length, digest );
    NSMutableString *result = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
    for(int i = 0; i < CC_MD5_DIGEST_LENGTH; i++)
        [result appendFormat:@"%02x", digest[i]];
    return result;
}